Propagation characteristics at mm-waves differ from the lower bands typically used in communication systems. The main characteristic is its “quasi-optical” behaviour due to the small wavelength size. Therefore, rough surface scattering effects are enhanced, reflections become more specular-like, and diffraction effects are very small. Furthermore, path loss is increased due to the high free-space attenuation. Penetration loss through building materials is significantly higher than at lower bands, limiting the indoor communication range mostly to intra-room applications. Due to such kind of penetration loss, non-line-of-sight (NLOS) transmission becomes more challenging than in lower bands.
The mm-wave channel is usually considered as “deterministic” since the fading effects introduced by the superposition of different paths are highly reduced in comparison to those in lower frequency bands. One illustration is that single paths can be well identified and isolated with different delays. Furthermore, due to its deterministic characteristic, each different path bears a clearly defined polarization. The polarization changes with the interaction of the waves with the environment. In particular, due to the small size of the wavelength, the number of objects which effect the propagation path by interacting and changing the polarization properties is increased. These changes can be the rotation of the linear polarization angle, for example by the cancellation of a determined polarization after the interaction with structures that only reflect one of the components of the polarization. Another consequence of the deterministic behaviour of the channel is that the environment acts as a spatial filter resulting in different parameters (e.g. Doppler and phase shift, delay, polarization) for every single path.
In order to counteract the high path-loss, mm-wave communication systems are expected to use high gain antennas, e.g. antenna arrays that can concentrate the energy in a desired direction. The relatively small size of mm-wave antennas allows the adoption of compact, very high-order MIMO arrays which enable narrow-beam beam-forming schemes. This provides a very high resolution in the directional domain.
Due to the high free-space propagation loss, wireless transmission in mm-wave frequency will mostly rely on beam forming. The key question is how to do beamforming to exploit the special characteristics of mm-wave channel and to maximize the receive signal quality. One of the main special characteristics of mm-wave channel propagation is its highly directional and spatial selectivity. Usually, there are only a limited number of propagation paths between the transmitter (TX) and the receiver (RX), including the Line-of-Sight (LOS) path and Non-Line-of-Sight (NLOS) paths. Due to the high directional selectivity of the mm-wave channel and the expected large bandwidth of the mm-wave system, these paths can be well resolved. For signal transmission, one can point the beams in direction of these paths.